in.telnetd: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/in.telnetd - from package: telnet-server

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/in.telnetd - from package: telnetd-ssl
/usr/sbin/in.telnetd - from package: telnetd

You may install the required package with command apt-get install {package}

Error in other languages:
in.telnetd: bevel nie gevind nie
in.telnetd: comandă negăsită
in.telnetd: comando não encontrado
in.telnetd: commande introuvable
in.telnetd: command not found
in.telnetd: käsku ei ole
in.telnetd: không tìm thấy lệnh
in.telnetd: komanda nerasta
in.telnetd: Kommando nicht gefunden.
in.telnetd: kommandot finns inte
in.telnetd: komut yok
in.telnetd: nie znaleziono polecenia
in.telnetd: níor aimsíodh an t-ordú
in.telnetd: no se encontró la orden
in.telnetd: no s'ha trobat l'ordre
in.telnetd: opdracht niet gevonden
in.telnetd: parancs nem található
in.telnetd: perintah tidak ditemukan
in.telnetd: príkaz nenájdený
in.telnetd: příkaz nenalezen
in.telnetd: команда не найдена
in.telnetd: командата не е открита
in.telnetd: 命令找不到
in.telnetd: コマンドが見つかりません